<!DOCTYPE html>
<html>
<head lang="en">
    <meta charset="utf-8">
    <meta name="viewport"
          content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0"/>
    <title>下拉菜单</title>
    <link href="css/bootstrap.css" rel="stylesheet">
    <style>
        body{
            padding: 10px;
        }
    </style>
</head>
<body>


    <br/>
    <br/>
    <br/>
    <br/>
    <br/>
    <br/>
    <br/>
    <br/>
    <br/>

    <div class="dropup open">  <!-- open控制菜单收缩展开-->
        <button class="btn btn-primary " data-toggle="dropdown" >
            Dropup
            <span class="caret"></span>    <!-- 向下小三角-->
        </button>
        <ul class="dropdown-menu">
            <li class="active"><a href="#">Action</a></li> <!-- active 默认是选中状态-->
            <li><a href="#">Another action</a></li>
            <li class="divider"></li>     <!-- 分界线-->
            <li><a href="#">Something else here</a></li>
            <li><a href="#">Separated link</a></li>
        </ul>
    </div>
</div>

<script src="js/jquery-1.12.4.js"></script>
<script src="js/bootstrap.js"></script>
</body>
</html>